1. Field of the Invention
This invention is in the field of removal of corrosion resistant, sol-gel derived, hybrid organic-inorganic coatings with application, among other things, in the aerospace industry.
2. Background
Organically modified silicate (Ormosil) coatings have been applied to various substrates and investigated for a diverse range of applications including enhancement of mechanical, thermal, optical, corrosion resistance and electrical properties of the underlying materials. Recent studies have shown that Ormosil films provide good corrosion protection for aluminum alloys as they form a dense, mechanically-stable, chemically-inert barrier layer on the metal surface. Ormosil coatings are of interest for aerospace applications due to their potential to replace hexavalent chromium conversion coatings as the corrosion inhibiting pretreatment.
For optimum corrosion protection, aircraft paint systems are removed and reapplied at 5-7 year intervals using abrasives or solvents. The former often results in damage to the metal substrate; the latter involves utilization of hazardous chemicals, such as methylene chloride. Implications of these difficulties associated with conventional coating removal techniques have prompted research for alternative cleaning methods such as coating pyrolysis, atmospheric-pressure pulsed-corona plasma, TEA-CO2 laser ablation, and UV laser irradiation. The high adhesion and chemical inertness of Ormosil coatings make their removal from the underlying metal substrate difficult using common paint or coating stripping techniques. For these reasons, it is desirable that a simple, non-toxic coating removal process for Ormosil coatings be developed.
It is commonly known that strongly basic solutions can be used to dissolve sol-gel derived materials. These solutions may also be used to dissolve sol-gel coatings on aluminum alloy substrates; however, these reagents may partially dissolve the underlying aluminum metal, especially at high pH values.
It is thus an object of the present invention to provide a simple, non-toxic method of removing Ormosil coatings without harming the underlying substrate material.
A process is provided for chemical removal of organically-modified silicate (Ormosil) coatings from aluminum alloy substrates without degradation of the underlying metal. Contact with a zincate solution, which is commonly used as a pretreatment in electroless or electroplated nickel deposition on aluminum alloys, dissolves the Ormosil film and deposits a thin, easily-removed layer of zinc onto the aluminum alloy surface, preventing base-activated dissolution of the underlying metal. If desired, the zinc layer may be removed using dilute phosphoric acid, leaving the surface of the aluminum alloy intact. In this manner, removal of the sol-gel coating is achieved while simultaneously maintaining the integrity of the aluminum alloy substrate.
It is believed that the invention described herein is the first demonstrated example of removing an Ormosil film from metal substrates using basic zincate solutions.
A better understanding of the present invention, its several aspects, and its advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached drawings, wherein there is shown and described the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention.